The use of weak acid resin to remove dissolved metals from wastewater is well known in the art. The resin comprises a multitude of tiny beads which are placed in a vertically extending container thereby forming a stacked column. The wastewater to be treated is moved through the resin column and the resin absorbs (takes up) the dissolved metal. The wastewater typically enters the top of the resin column and exits at the bottom. The resin beads are prevented from being swept out of the container by a strainer that is located on the downstream end or effluent end of the column.
The general structure of a weak acid resin is typically polyacrylic or polymethacrylic. These two resins are populated by chemically functional groups called carboxylic acids. A carboxylic acid is of the same family as acetic acid and is typically written as --COOH. This is a weak acid as opposed to strong acids such as sulfonic acid (typically written --SOOOH). Both acids will lose their hydrogen ion or proton (H.sup.+), a process called deprotonation as shown in the following equations:
--COOH.fwdarw.--COO.sup.- +H.sup.+, and --SOOOH.fwdarw.--SOOO.sup.- +H.sup.+ [reaction 1]
The stronger the acid, the greater is the tendency for the above reaction to proceed. The carboxylic group will only undergo the above reaction at about a pH of approximately 5.5 or greater. The sulfonic group, on the other hand, will undergo the reaction at any pH above about 0.5. This limitation of the weak acid is its hallmark.
To prepare a weak acid resin for removing metal salt, the resin must undergo reaction 2 below. This is typically done by contacting the resin with a sodium hydroxide solution, but can be done with any basic material such as ammonium hydroxide, calcium hydroxide, sodium carbonate, etc. Sodium hydroxide, for example, creates a very high pH condition in the water around the resin, which promotes reaction 2 below. EQU --COOH+NaOH.fwdarw.--COO.sup.- Na.sup.+ +H.sub.2 O [reaction 2]
Dissolved metal ions (written as M.sup.++) can subsequently be taken up by the resin according to the following reaction: EQU 2(--COO.sup.- Na.sup.+)+M.sup.++ 43 2(--COO.sup.-)M.sup.++ +2 Na.sup.+ [reaction 3]
Since the resin is a weak acid and not a strong acid, reaction 4 below cannot proceed because the pH of the surrounding water is too low. Another reason why reaction 4 cannot proceed in the direction indicated is that he species of acid created in the surrounding solution by the hypothetical deprotonation of the carboxylic group would be stronger than carboxylic acid group itself. This is a violation of the second law of thermodynamics. EQU 2(--COOH)+M.sup.++.fwdarw.2(--COO.sup.-)M.sup.++ +2 H.sup.+ [reaction 4, won't proceed]
In other words, if the functional groups on the resin are filled with protons (H.sup.+) instead of sodium ions (Na.sup.+) or calcium ions (Ca.sup.++) reaction 3 cannot happen and the resin can take up no more metal ions.
Unfortunately metal-bearing wastewater typically has a pH between 1 and 5. When weak acid (instead of strong acid) resin comes in contact with many gallons of this acidic water, the carboxylic function groups on the resin will be converted from the sodium form to the proton form as follows: EQU --COO.sup.- Na.sup.+ +HCl.fwdarw.--COOH+Na.sup.+ Cl.sup.- [reaction 5]
In reaction 5, the HCl acid is neutralized by the sodium form carboxylic group and is converted to the proton form in the process. The by-product is sodium chloride. Reaction 5 is what creates a neutral pH water in the effluent. As more of this acidic water flows through the resin more of the sodium ions are stripped-off the resin and replaced with protons. It can also be shown that the metals on the resin are pushed to the outlet end of the resin column (refer to FIG. 2). Finally, a point is reached when there are no more sodium ions on the resin and reaction 3 or 5 cannot proceed. Instead reaction 4 will occur--but in reverse--and leakage of metal ions will occur in the effluent. At this point, it has been typically assumed that the resin must be regenerated to remove the metals. This is done by adding a strong acid to promote reaction 4 in reverse, which is quite thermodynamically favorable when done with a strong acid. After the resin has been regenerated or placed in its proton form, it can be converted to its sodium ion form by adding an alkaline material such as sodium hydroxide or sodium carbonate in a process called reneutralizing.
As will be seen in the following disclosure and claims, the method of the present invention places the resin in its reneutralized form a plurality of times without first regenerating with acid.